Fetal tissue, and in particular fetal DNA and chromosomes, is routinely used in prenatal diagnosis and other medical procedures that require an accurate assessment of the genome of the fetus. Currently, the fetal tissue is obtained by the use of amniocentesis, chorionic villus sampling (CVS), fetoscopy, or cordocentesis, as described in Thompson and Thompson Genetics in Medicine, 5th Edition, W. B. Saunders Co., Philadelphia, 1991.
In amniocentesis, a sample of amniotic fluid, which contains fetal cells, is transabdominally removed from the mother with a needle and syringe. Amniocentesis has inherent associated risks. The major risk is induction of miscarriage which is estimated to occur in 1 in 200 amniocenteses. Other risks include maternal infection and physical damage to the fetus. In CVS, trophoblast tissue is aspirated from the villous area of the chorion transcervically or transabdominally. The rate of fetal loss by this method may be as high as 1 in 100. Cordocentesis or percutaneous umbilical blood sampling provides a method of obtaining fetal blood directly from the umbilical cord with ultrasonic guidance. Each of these invasive methods carries risks to both the mother and the fetus.
Accordingly, it would be desirable to have a noninvasive method for obtaining fetal tissue or fetal DNA. It would also be desirable to have a method that is rapid and reliable for isolating and enriching the fetal tissue from maternal tissue in order to facilitate screening and pre-natal diagnosis in clinical laboratories. Recently, the preferred methodology has been the identification of fetal cells in the peripheral maternal circulation and then garnering of those cells for genetic analysis.
Identification or isolation of fetal cells from maternal blood has relied upon distinguishing the rare population of fetal cells from the more prevalent maternal cells. Although various fetal cell types, such as fetal lymphocytes and trophoblasts, have been utilized in the identification process as target cells for fetal DNA, more efforts have been directed to fetal nucleated red blood cells (nRBC), also known as nucleated erythrocytes. See Cheuh and Golbus, "The Search for Fetal Cells in the Maternal Circulation", J. Perinatol. Med., 19:411 (1991); Simpson, et al., "Noninvasive Screening for Prenatal Genetic Diagnosis", Bull. WHO, 73:799 (1995); and Cheuh and Golbus, "Prenatal Diagnosis Using Fetal Cells from Maternal Circulation", West. J. Med., 159(3):308 (1993).
Fetal RBCs are thought to cross the placenta as a result of transplacental bleeding. Since the fetus has a large number of nucleated erythrocytes, which nucleated erythrocytes are rarely found in adult blood, the difference in nucleation is useful in separating and identifying fetal cells from maternal ones.
Antibodies to cell surface antigens particular to nRBCs, such as the transferrin receptor, have been utilized to identify and enrich for these fetal cells. See Bianchi, et al., "Isolation of Fetal DNA from Nucleated Erythrocytes in Maternal Blood", Proc. Natl. Acad. Sci, 87:3279 (1990). See also Bianchi, et al. PCT International Application No. PCT/US90/06623 (WO 91/07660), which describes a method for enriching fetal nucleated red blood cells from a peripheral blood sample by the use of an antibody which binds an antigen on the cell surface of the fetal cells.
Bresser, et al., PCT International Application No. PCT/US94/08342 (WO 95/03431) describes the use of fetal hemoglobin antibodies and mRNA probes to enrich for fetal cells in maternal blood. The presence of fetal hemoglobin also has been demonstrated by the Kleihauer-Betke reaction that differentiates fetal from adult hemoglobin by acid elution characteristics. See Kleihauer, et al., "Demonstration von fetalem hamoglobin in den erythrocyten eines blutausstrichs", Klin. Woschenschr, 35:637 (1957); and Saunders, et al., "Enrichment of fetal cells from maternal blood for genetic analysis", American Journal of Human Genetics,57:287 (1995).
Genetic analysis of the fetal genome has been accomplished by fluorescence in situ hybridization (FISH) of chromosome or gene specific DNA or RNA probes, sometimes with automated reading, and by amplification of targeted fetal genes or DNA. See Lichter, et al., "Rapid detection of human chromosome 21 aberration analysis using fluorescence in situ hybridization", Proc. Natl. Acad. Sci., 85:9664 (1988); O'Kelley, et al., "Instrumentation for the genetic evaluation of fetal cells from maternal blood", Am. J. Hum. Genet., 57:286 (1995); and Lo, et al., "Prenatal Sex Determination by DNA amplification from maternal peripheral blood", Lancet, 2:1363 (1989).